Quanlin Zhang

High-performance zero-bias ultraviolet photodetector based on p-GaN/n-ZnO heterojunction

Lattice-match p-GaN and n-ZnO bilayers were heteroepitaxially grown on the c-sapphire substrate by metal organic chemical vapor deposition and molecular beam epitaxy technique, respectively. X-ray diffraction and photoluminescence investigations revealed the high crystal quality of the bilayer films. Subsequently, a p-GaN/n-ZnO heterojunction photodetector was fabricated. The p-n junction exhibited a clear rectifying I-V characteristic with a turn-on voltage of 3.7 V. At zero-bias voltage, the peak responsivity was 0.68 mA/W at 358 nm, which is one of the best performances reported for p-GaN/n-ZnO heterojunction detectors due to the excellent crystal quality of the bilayer films. These show that the high-performance p-GaN/n-ZnO heterojunction diode is potential for applications of portable UV detectors without driving power.

Formation behavior of BexZn1−xO alloys grown by plasma-assisted molecular beam epitaxy

We report the phase formation behavior of BexZn1−xO alloys grown by plasma-assisted molecular beam epitaxy. We find the alloy with low- and high-Be contents could be obtained by alloying BeO into ZnO films. X-ray diffraction measurements shows the c lattice constant value shrinks, and room temperature absorption shows the energy band-gap widens after Be incorporated. However, the alloy with intermediate Be composition are unstable and segregated into low- and high-Be contents BeZnO alloys. We demonstrate the phase segregation of BexZn1−xO alloys with intermediate Be composition resulted from large internal strain induced by large lattice mismatch between BeO and ZnO.

Temperature-dependent structural relaxation of BeZnO alloys

The thermal stability of BeZnO was examined in this study. Structural relaxation and reconstruction of the Be0.4Zn0.6O alloy film started at 500 °C and came to a halt at 800 °C. Be atoms were found to be diffused out from the host position, and BeO-based second phase was formed. The bandgap of Be0.4Zn0.6O was reduced to the value of pure ZnO after annealing at 600 °C. Therefore, the thermodynamic solubility of BeO in ZnO is far below than that of MgO in ZnO. Finally, the long term stability of BeZnO at room temperature was verified after aged for one year.

Solar-blind wurtzite MgZnO alloy films stabilized by Be doping

MgxZn1?xO alloy films were deposited on c-plane sapphire substrates by radio frequency plasma-assisted molecular beam epitaxy (rf-PMBE). The phase segregation occurred when x was larger than 33%. Be doping was found experimentally able to stabilize the high-Mg-content MgZnO alloy. By alloying 1?2% Be into MgZnO, the band gap of as-prepared quaternary alloys can be raised to the solar-blind range (4.5?eV). Calculated formation energy of the alloys based on first principle reveals that a small amount of Be incorporation can reduce the formation energy of high-Mg-content MgZnO alloys and results in a more stable system, which justifies our experimental observations.

Stabilization of p-type dopant nitrogen in BeZnO ternary alloy epitaxial thin films

We demonstrate the growth of BexZn1−xO alloy thin films on c-sapphire substrates by plasma-assisted molecular beam epitaxy. The formation of BexZn1−xO is very sensitive to the Be content in the alloy. Be atoms occupy the Zn lattice sites at low Be content, but move partly to the interstitial sites with increasing Be content. We also investigated the thermal stability of N, one of the most frequently used p-type dopants, in the BexZn1−xO films. Secondary ion mass spectrometry shows that Be element plays a crucial role in stabilizing N in the BexZn1−xO host, which is favourable in improving the solid solubility and the thermal stability of acceptor dopants in ZnO-based wide-gap semiconductors.